First, try purging cache via URL line in browser:
http://yoursite/admin/purgecaches.php
That will force you to login first before executing the script.
If you can't do that, how about command line:
cd moodlecode/admin/cli/
php purge_caches.php [ENTER]
If that is not possible, edit the config.php file of the site and add the following line:
$CFG->theme ='standard';
No need to restart apache or any service/server, just hit site with browser.
Login. Purge Caches. Logout.
Re-edit the config.php file and comment out the line you added above by adding // in front. As long as a CFG line is in config.php and active it overrides DB settings.
Now try logging in again with the theme you chose via Moodle UI.
If it messes up again, redit config.php un-comment the theme line again and consider adding debugging lines.
'spirit of sharing', Ken